In the following, the structure of a solid oxide cell stack is explained in relation to fuel cells. The fuel cells may however also run in “reverse mode” and thus operate as electrolysis cells.
A Solid Oxide Fuel Cell (SOFC) comprises a solid electrolyte that enables the conduction of oxygen ions, a cathode where oxygen is reduced to oxygen ions and an anode where hydrogen is oxidised. The overall reaction in a SOFC is that hydrogen and oxygen electrochemically react to produce electricity, heat and water. In order to produce the required hydrogen, the anode normally possesses catalytic activity for the steam reforming of hydrocarbons, particularly natural gas, whereby hydrogen, carbon dioxide and carbon monoxide are generated. Steam reforming of methane, the main component of natural gas, can be described by the following equations:CH4+H20→CO+3H2 CH4+CO2→2CO+2H2 CO+H20→CO2+H2 
During operation an oxidant such as air is supplied to the solid oxide fuel cell in the cathode region. Fuel such as hydrogen is supplied in the anode region of the fuel cell. Alternatively, a hydrocarbon fuel such as methane is supplied in the anode region, where it is converted to hydrogen and carbon oxides by the above reactions. Hydrogen passes through the porous anode and reacts at the anode/electrolyte interface with oxygen ions generated on the cathode side that have diffused through the electrolyte. Oxygen ions are created in the cathode side with an input of electrons from the external electrical circuit of the cell.
To increase voltage, several cell units are assembled to form a stack and are linked together by interconnects. Interconnects serve as a gas barrier to separate the anode (fuel) and cathode (air/oxygen) sides of adjacent cell units, and at the same time they enable current conduction between the adjacent cells, i.e. between an anode of one cell with a surplus of electrons and a cathode of a neighbouring cell needing electrons for the reduction process. Further, interconnects are normally provided with a plurality of flow paths for the passage of fuel gas on one side of the interconnect and oxidant gas on the opposite side. To optimize the performance of a SOFC stack, a range of positive values should be maximized without unacceptable consequence on another range of related negative values which should be minimized. Some of these values are:
VALUES TO BE MAXIMIZEDVALUES TO BE MINIMIZEDFuel utilizationPriceelectrical efficiencyDimensionslife time(temperature, to a point)production timefail ratenumber of componentsParasitic loss (heating,cooling, blowers.)
Almost all the above listed values are interrelated, which means that altering one value will impact other values. Some relations between the characteristics of gas flow in the fuel cells and the above values are mentioned here:
Fuel Utilization:
The flow paths on the fuel side of the interconnect should be designed to seek an equal amount of fuel to each cell in a stack, i.e. there should be no flow-“short-cuts” through the fuel side of the stack.
Parasitic Loss:
Design of the process gas flow paths in the SOFC stack and its fuel cell units should seek to achieve a low pressure loss per flow volume at least on the air side and potentially on the fuel side of the interconnect, which will reduce the parasitic loss to blowers.
Electric Efficiency:
The interconnect leads current between the anode and the cathode layer of neighbouring cells. Hence, to reduce internal resistance, the electrically conducting contact points (hereafter merely called “contact points”) of the interconnect should be designed to establish good electrically contact to the electrodes (anode and cathode) and the contact points should no where be far apart, which would force the current to run through a longer distance of the electrode with resulting higher internal resistance.
Lifetime:
Depends in relation to the interconnect on even flow distribution on both fuel and air side of the interconnect, few components and even protective coating on the materials among others.
Price:
The interconnects price contribution can be reduced by not using noble materials, by reducing the production time of the interconnect and minimizing the material loss.
Dimensions:
The overall dimensions of a fuel stack is reduced, when the interconnect design ensures a high utilization of the active cell area. Dead-areas with low fuel- or air flow should be reduced and inactive zones for sealing surfaces should be minimized.
Temperature:
The temperature should be high enough to ensure catalytic reaction in the cell, yet low enough to avoid accelerated degradation of the cell components. The interconnect should therefore contribute to an even temperature distribution giving a high average temperature without exceeding the maximum temperature.
Production Time.
Production time of the interconnect itself should be minimized and the interconnect design should also contribute to a fast assembling of the entire stack. In general, for every component the interconnect design renders unnecessary, there is a gain in production time.
Fail Rate.
The interconnect production methods and materials should permit a low interconnect fail rate (such as unwanted holes in the interconnect gas barrier, uneven material thickness or characteristics). Further the fail-rate of the assembled cell stack can be reduced when the interconnect design reduces the total number of components to be assembled and reduces the length of seal surfaces.
Number of Components.
Apart from minimizing errors and assembling time as already mentioned, a reduction of the number of components leads to a reduced price.
The way the anode and cathode gas flows are distributed in an SOFC stack is by having a common manifold for each of the two process gasses. The manifolds can either be internal or external. The manifolds supply process gasses to the individual layers in the SOFC stack by the means of channels to each layer. The channels are normally situated in one layer of the repeating elements which are comprised in the SOFC stack, i.e. in the spacers or in the interconnect.
When operating a SOC stack, connections to the stack are necessary. It is at least necessary to have process gas connections and electrical connections. Manifolds and piping are used to connect a stack with process gas. In some embodiments, it is necessary to apply gaskets between the manifolds and piping, and the SOC stack.
As SOC stacks operate at high temperatures often above 700° C., the gaskets need to be able to withstand multiple thermal cycles and still be leak proof. It is a requirement to the SOC stacks that they can be electrically connected in series and that they are electrically floating, i.e. none of the stacks are electrically grounded. Therefore the gaskets need also to be electrically insulating.
US2005266288 discloses a solid oxide fuel cell generator that contains stacks of hollow axially elongated fuel cells having an open top end, an oxidant inlet plenum, a feed fuel plenum, a combustion chamber for combusting reacted oxidant/spent fuel; and, optionally, a fuel recirculation chamber below the combustion chamber, where the fuel recirculation chamber is in part defined by semi-porous fuel cell positioning gasket, all within an outer generator enclosure, wherein the fuel cell gasket has a laminate structure comprising at least a compliant fibrous mat support layer and a strong, yet flexible woven layer, which may contain catalytic particles facing the combustion chamber, where the catalyst, if used, is effective to further oxidize exhaust fuel and protect the open top end (37) of the fuel cells.
US2006121327 describes a solid-oxide fuel cell assembly comprising a plurality of components having electrically-conductive mating surfaces there between, the surfaces are sealed by an electrically insulating gasket that include a mineral composition comprising about 66 mol percent MgO and about 33 mol percent SiO2, the mineral composition being known mineralogically as forsterite. A brazing alloy may be applied to enhance bonding of the gasket into place. The gasket composition may include additions of Al2O3 to enhance electrical resistivity while having little to no impact of matching expansion coefficients of the gasket and metal mating surfaces. Also, additions such as titania or zirconia to inhibit glassy phase grain boundaries and the formation of impurities and pores in the ceramic grain boundaries may be used. A recommended particle size distribution of precursor powders is disclosed that leads to an optimum microstructure of the sintered gasket.
None of the above described known art provides a simple, efficient and fail-safe solution to the above described problems.
Therefore, with reference to the above listed considerations, there is a need for a robust, simple, cheap and easy to produce and handle gas tight, temperature resistant, electrically insulating and vibration resistant gasket for a solid oxide fuel cell stack system. As corresponding cell stack systems can also be used for solid oxide electrolysis, this gasket solution can also be used for a SOEC stack system; hence a solution is sought for a SOC stack system.
These and other objects are achieved by the invention as described below and in the claims.